The present invention relates to a high speed modem used for data transmission and in particular to a modem which allows two independent multipoint networks to share the same communications lines or media.
Multiplexing in frequency or in-time permits multiple users to share a common communication media simultaneously. In Communication System Engineering Handbook by D. H. Hamsher, McGraw-Hill, 1967, various data transmission multiplexing techniques are discussed in detail. These techniques assume that the multiple users and hence the multiplexing equipment are connected in a point-to-point fashion by the communication media.
Recently many communication systems have been developed which employ a multi-point or multi-drop configuration. In these systems, many users share a common party line to a central site. While the central site may broadcast to all users, at any one time only one user may transmit to the central site. If two users transmit to the central site simultaneously, they mutually interefer and both transmissions are lost. The protocol employed to insure proper user response is referred to as polled. A typical multi-point communication system is depicted in FIG. 1 in which the communications line is a telephone line. The operation of the depicted multi-point or, as it is sometimes called, multi-drop system is discussed in detail in Systems Analysis for Data Transmission by James Martin, Prentice-Hall, Inc., 1972. It suffices for the present discussion to note that multiple terminals and associated modems (1, 2, and 3, for example) are connected through a telephone party line to a master modem which in turn is connected to the system front end processor.
With the advent of large, complex data communication systems, many users have found a need for two independent multi-point communication systems which serve, or are located at common sites. Previously, this would require twice the hardware and components of a one multipoint system. That is, a dual system requires two sets of communication lines and two modems at each site, one for each of the terminals. FIG. 2 illustrates such a dual system in which it should be noted that at each location 1, 2 and 3 there is an "A" modem and a "B" modem and a connection with an "A" and a "B" party line.
The obvious disadvantage of the dual system of FIG. 2 is that it requires double the outlay for equipment as would be required for the system of FIG. 1. In addition, double the rental fees must be paid for the two lines required as compared to the single line of FIG. 1.
In view of the above, it is the principal object of the present invention to provide a multi-point system which permits the virtual independent operation of two multipoint communication systems through a single modem at each location.
A further object of this invention is to provide a transmitter/receiver structure which allows virtual independent operation of each of the multi-point systems over the same communication line.
Other objects and advantages will be self-evident from the description of the preferred embodiment of the invention.